Temporary road mats are used extensively to allow vehicles and equipment to reach locations not serviced by permanent roads, particularly under conditions where ground surface would not support the traffic or where the temporary nature of use does not warrant the time and expense of a permanent road. The mats are generally constructed in multiple layers of heavy wood timbers in crisscrossing layers. The mats may also be provided with interlocking joints for connecting with adjacent mats as disclosed in U.S. Pat. No. 4,462,712 to Penland; U.S. Pat. No. 5,032,037 to Phillips et al.; and U.S. Pat. No. 4,889,444 to Pouyer. Commonly, each layer is nailed to the underlying layer. The final layer is fastened with an extended length nail having a protruding tip that crimped over to secure the assembly as disclosed in U.S. Pat. No. 5,234,204 to Hunt. The manual assembly, even with air powered nailing guns in time consuming and tiring and prone to misalignments. For heavy duty applications, three ply mats are employed. Initially, two layers are nailed as a subunit, with the third layer nailed thereto in a separate assembly sequence. This type of nailed construction provides limited torsional and bending strength, resulting loosening of the nailed joints and timber breakage. Replacing a damage timber requires time consuming removal of all overlying timbers. The entire length of timber must be replaced inasmuch as limited compression and shear strength precludes splicing with shorter lengths. Where the nail loosening occurs at the intermediate layer, supplemental nailing from both the top and bottom surfaces may be required to regain suitable strength at the joint. Such supplemental nailing can also splinter the timber resulting in a complete loss of nail holding strength.
Various assembly devices have been proposed for more accurately aligning the timber layers prior to nailing. U.S. Pat. No. 4,922,598 to Pouyer discloses an assembly table having an array of alignment stops for assisting positioning the timbers prior to nailing. U.S. Pat. No. 5,234,204 to Hunt, referenced above, discloses a dual stage assembly table facilitating nailing on opposite sides of the mat. Crimped extended length nails are used on the final layer to resist a lifting of the nail heads that can damage vehicle tires.
The nailing of the timbers also precludes convenient repair and replacement of damaged timbers. Typically, the mat must be substantially disassembled to access and replace damaged members. Bolted constructions have been suggested in the art, however, such mats are not known to be commercially available. Therein, holes are drilled at the intersections of the crossed timbers for receiving threaded fasteners. The resulting construction provides a smooth upper surface free of protruding nail heads. When replacement of a member is required, only the fasteners in the affected area need to be removed to allow substitution. Notwithstanding the advantages of the bolted construction, production rates, even with fixtures and assembly tables, have been limited by the cycle times of the serially required operations.
Moreover, three ply mats have not been successfully produced. The cumulative tolerances in the hole arrays for the three layers present's extreme alignment problems for assembly. Approaches increasing the clearances between the holes and bolt shanks result in a loss of lateral structural stability. Leveraging the boards to align the holes for assembly results in a stressed assembly condition that can result in timber fracture and surface warping.
Accordingly, a need continues to exist for three ply bolted temporary road mats that may be assembled accurately under low stress conditions.